The invention relates to electronic semiconductor devices and circuits, and, more particularly, to methods of fabrication with epitaxial overgrowth and devices so fabricated.
Various semiconductor processing flows include formation of epitaxial layers on substrates. Such epilayers may provide a change in doping level, a crystal superior to the substrate's, or even a change in material such as a gallium arsenide (GaAs) substrate with an aluminum gallium arsenide (Al.sub.x Ga.sub.1-x As) epilayer for heterojunction devices. Epilayer growth by metalorganic chemical vapor deposition (MOCVD) on {100} oriented GaAs substrates produces smoother surfaces when the substrate orientation is tilted 2.degree. in the {110} direction, and industry standard GaAs wafers have such an orientation tilt. This tilt provides a slightly terraced surface (terrace widths on the order of 10-20 nm) which apparently insures smooth epilayer growth.
Certain processing flows include epitaxial overgrowth of nonplanar structures. In particular, silicon bipolar transistor processes frequently have a buffed layer formed by epitaxial overgrowth of a doped region which has been depressed below the substrate surface by an oxidizing drive-in cycle. But more significantly, heterojunction bipolar transistors (HBTs) and self-aligned structure (SAS) lasers can be fabricated with epitaxial growth over steps in a GaAs layer. See Plumton et al, Planar AlGaAs/GaAs HBT Fabricated by MOCVD Overgrowth with a Grown Base, 37 IEEE Trans. Elec. Dev. 118 (1990)(growth of n-Al.sub.x Ga.sub.1-x As emitter over p-GaAs base mesa for an HBT) and Noda et al, Effects of GaAs/AlAs superlattic buffer layers on selective area regrowth for GaAs/AlGaAs self-aligned structure lasers, 47 Appl. Phys. Lett. 1205 (1985)(molecular beam epitaxy growth of p-Al.sub.x Ga.sub.1-x As over n-GaAs antiguiding mesa for a SAS laser). However, such epitaxial overgrowth on step structures has problems including finding growth conditions for enhancing device performance.
Solar cells made of GaAs provide radiation hardness and find use as power supplies for spacecraft and earth communication satellites. However, GaAs substrates have problems including a lack of strength, so growing GaAs on germanium (Ge) has been of interest for solar cells. For example, Iles et al, High-Efficiency (&gt;20% AM0) GaAs Solar Cells Grown on Inactive-Ge Substrates, 11 IEEE Elec.Dev.Lett. 170 (1990), and Chen et al, GaAs/Ge Heterojunction Grown by Meatl-Organic Chemical Vapor Deposition and its Application to High Efficiency Photovoltaic Devices, 21 J.Elec.Mat. 347 (1992), disclose GaAs p-n junction solar cells situated on Ge substrates.